Medical microwave radiometers are instruments that measure thermally generated microwave emissions from subsurface tissues in order to determine the temperature of the tissues. Radiometers can non-invasively measure subsurface tissue temperatures to a depth of several centimeters. By contrast, conventional instruments for measuring subsurface tissue temperatures, such as thermocouple or thermistor thermometers, fiber optic thermometers, etc., each employ an invasive probe that is inserted into the tissues whose temperature is to be measured. Still other known instruments, such as non-invasive IR (infrared) thermometers, measure only skin surface temperature without producing any meaningful information regarding subsurface tissue temperatures. The non-invasive feature of microwave radiometers makes them attractive candidates for measuring subsurface tissue temperatures in a number of medical applications.
One type of microwave radiometer uses the so-called “Dicke” method. The Dicke method causes the input of a microwave receiver to be alternately switched by a Dicke switch at some rate, between a reference noise source and noise which is received by a microwave antenna. A modulator controls both the operation of the Dicke switch and a synchronous detector, which in conjunction with a rectifier forms a synchronous demodulator. One exemplary radiometer using the Dicke method was described by Land in “An efficient, accurate and robust radiometer configuration for microwave temperature measurement for industrial and medical applications”, Journal of Microwave Power and Electromagnetic Energy, 36 (3), pgs 139-153, 2001. Land's radiometer connected a main chassis having a microwave switching amplifier by cable to a remote microwave antenna. As is typical of Dicke radiometers, Land's radiometer required synchronous source-reference switching coupled to a synchronous demodulator in the main chassis.
The inventors of the present application, Sterzer and Mawhinney, previously developed a microwave radiometer for the MMTC of Princeton, N.J. FIG. 1 shows a block diagram of one embodiment of the earlier MMTC radiometer, labeled 10. This prior radiometer presented a significant advantage over the Dicke method, in that the synchronous reference-source switching of the Dicke switch or the synchronous demodulator was no longer required. However, the microwave radiometer 10 of FIG. 1 still required five separate microwave switches 62. Beyond a dual temperature reference termination 32 at one port of a circulator 20, the radiometer 10 also had to sequence through two more heated terminations 34, 38 on the antenna side of the reference plane 40, as well as to continuously select between a short 44 at the reference plane 40 and cables to a remote antenna, as well as continuously selecting between a shorted “dummy” cable and the actual microwave signal source from the remote receiving antenna 18. While capable of relatively high performance and used for applications ranging from hyperthermia treatments to basic research in medical radiometry (e.g. as reported by Arunachalam, Sterzer (one of the inventors), et al., in “Characterization of a digital microwave radiometry system for noninvasive thermometry using a temperature-controlled homogeneous test load”, PHYSICS IN MEDICINE AND BIOLOGY, Phys. Med. Biol. 53, pages 3883-3901, July, 2008), the radiometer of FIG. 1 is an example of a still relatively complex and expensive medical radiometer instrument.
What is needed, therefore, is a simplified and less costly medical radiometer, which still determines subsurface temperatures, such as, for example, core body temperature, in a reliable and cost effective manner.